The present invention relates to a color imaging system and more particularly to an improved imaging system using two charge coupled device (CCD) image sensors for producing color information signals in response to light received from an object. The disclosed color imaging system may be used as part of a color television camera, particularly, a small hand held color television camera.
Conventional color television camera systems generate the three color signals, e.g., red, green and blue signal, using three separate vidicon tubes. Splitting the light coming from the object to be televised into three primary colors for transmission to the three vidicons and the magnetic or electrostatic deflection systems associated with each vidicon have resulted in the prior color TV cameras being relatively large and heavy. While this prior art system produces a high quality color signal, the bulk and weight of the camera associated with this system makes it difficult to use, for instance, in hand held applications.
In the prior art there have been suggestions for color television image pickup systems which utilize, for instance, a striped optical filter and one or two vidicons, or similar devices such as the "plumbicon." Since white and colored light are reproducable using three primary colors, these one or two vidicon systems must be able to discern three primary colors. Typically, a striped optical filter is disposed in front of the vidicon interpreting two or three primary colors. The striped filter causes an alternating pattern of narrow stripes of different color light from the object to fall upon the vidicon's sensitive surface. These stripes are selected sufficiently narrow such that the number of different segments or portions of the output signal from the vidicon, corresponding to striped pattern of light falling on the vidicon's sensitive surface, provide sufficient resolution to yield a clear picture.
The magnetic or electrostatic deflection system used with a vidicon, or similar device, causes its electron beam scan or interrogate its light sensitive surface to produce an output signal; however, its electron beam does not move with constant speed across the light sensitive target. While the output signal contains signal pulses corresponding to the intensity of light falling along the electron beam scan line, there is an inherent uncertainty as to the particular color of light triggering a particular pulse. This uncertainty is caused by a combination of the non-constant speed of interrogation along a scan line and the lack of means for determining which color of light produced a particular output pulse. This uncertainty makes it difficult for the decoding circuitry to discern which segments of the output signal correspond to a particular primary color. Complex circuits have been utilized to compensate for this uncertainty. For instance, by using a parallel striped filter having a different pitch stripe for each color, color information is then associated with different carrier frequencies (in the output signal) corresponding to the pitch of the stripes, which information can be separated into separate color information signals using band pass filters. Alternatively, the prior art suggests disposing the stripes at angles and using phase demodulation techniques to separate the color information signals. In either case, the decoding circuitry is complex and because of bandwidth limitations of the vidicon, they are not totally satisfactory. These two vidicon systems used in the prior art incorporate such complex circuitry to compensate aforementioned scanning uncertainties in decoding the color signals generated by the vidicons.
In the prior art it has also been proposed that a charge coupled device (CCD) be used as an image sensor. For instance, U.S. Pat. No. 3,801,884 issued Apr. 2, 1974, discloses the use of a CCD array in an image pickup system. CCD arrays having 512 by 320 cells for use as an image sensor are currently commercially available from Radio Corporation of America, Lancaster, PA. under the tradename SID51232. Image sensing CCDs are made up of an array of charged coupled cells which use a clock rather than a scanning electron beam to scan or interrogate the cells. Since highly accurate clocks are known in the prior art, the CCD image sensor does not suffer the non-linearity associated with the magnetic or electrostatic deflection system of a conventional vidicon. That is, a particular output pulse from the CCD image sensor can be associated with a particular physical location on the image sensing surface of the CCD.
There has also been a proposal to use a single CCD image sensor in a color image pickup system, as is described in U.S. patent application Ser. No. 502,289, dated Jan. 13, 1976. This system utilizes a complex optical filter whose filter elements are arranged in an array and are sized approximately one-half the size of the CCD elements in the CCD array. This system further provides merely one of two color information signals for each scan line in sequential fashion in addition to a luminance signal for each scan line.
It is, therefore, an object of this invention to improve color television image pickup systems.
It is yet another object of this invention to reduce the complexity of color television pickup systems, for example, by using simplified circuits to decode the signals from two CCD image sensors, to produce three color signals suitable for use in broadcasting a color image of the object.
It is still another object of this invention to use a striped optical filter in conjunction with two CCD image sensors to produce three color signals suitable for use in broadcasting a color image of an object.
It is still yet another object of this invention to simplify striped filters used in two sensor color imaging systems.
It is another object of this invention to produce the color information signal during each scan line producing video information.
It is still another object of this invention to reduce the size of color television camera systems.
It is still yet another object to produce a color television camera system of lower cost than the systems generally in use in the prior art.
The foregoing objects are achieved according to the present invention as is now described. In a preferred embodiment of the invention, two CCD image sensors are utilized. Green light from an object is directed to the first image sensor while combined red and blue light is directed to the second image sensor by a partially reflective mirror. A striped optical filter is disposed between the light source and the second image sensor such that alternating stripes of red and red + blue light are imaged on the second image sensor's sensitive surface. The output from the red and red + blue image sensor is converted into separate red and blue color information signals by examining the pulses or segments of the image sensor signal corresponding to red stripes of light to generate the red color information signal and by examining the pulses or segments of the image sensor signal corresponding to red + blue stripes of light and substracting the pulses or segments of the signal corresponding to adjacent red stripes therefrom to produce the blue color information signal. Alternate embodiments disclosed use different combinations of primary colors to produce three color information output signals and use a striped filter which passes alternating stripes of red and blue light.